Optical magnetic field sensor

Abstract: The invention refers to an optical magnetic field sensor in which a polarizer, a magneto-optical element and an analyzer whose transmission-polarization direction is made different from that of said polarizer are arranged along at least a light propagating direction to detect as an output light intensity a magnetic field to be measured, wherein a transmission path diameter of a second optical transmission line provided across said analyzer at an end of said magneto-optical element is larger than a transmission path diameter of a first optical transmission line provided across said polarizer at the other end of said magneto-optical element.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical element having the Faraday effect and an optical magnetic field sensor composed by the use of the element. 2. Description of Prior Art Example Recently, in particular, in the field of power industry, there has been proposed and commercially applied a magnetic field measuring equipment combining a magneto-optical element having the Faraday effect with an optical fiber, as a method of measuring a magnetic field intensity developing around an electric wire by the use of a light. The method of measuring a magnetic field intensity around a conductor through which a current flows to detect the current is characterized in that, for example, the method has good insulation properties because it uses the light as a medium, and is not subjected to an electromagnetic induction noise, so that the method is considered to be used for transmission/distribution facilities. EP086387 discloses a magnetic field measuring instrument utilizing Faraday rotation, comprising a polarizer, a magneto-optical material and an analyseur. Furthermore US4554449 discloses a fibre optic magnetic field sensor also employing the Faraday effect. The sensor has two pieces of birefringent substance which are interposed between the ends of an optical fibre and a rod lens for effecting polarisation separation. In addition, the sensor comprises a reflex optical system in which the light reciprocates through Farraday material. JP02281169 discloses another optical magnetic sensor comprising a polarizer, a rod lens, a magneto optic member 6 and a comer cubed mirror 7 used so that the impinging light travels twice through the magneto optic material, then becomes condensed by a second rod lens 4 and transmits through the polarizer 5 again into a second fibre 2 which is thicker than the first fibre 1. Furthermore JP02135416 proposes to use a Fresnel lens to collimate a laser beam. Fig. 3 shows a principle diagram of a method of measuring a magnetic field by the use of the Faraday effect. In Fig. 3, a magneto-optical element 14 is arranged in a magnetic field H. An incident light 8a turned to a linearly polarized light by a polarizer 13 is caused to pass through the magneto-optical element 14. The plane of polarization thereof is rotated in proportion to the magnetic field intensity H by the Faraday effect. Fig. 3 shows a case where the Faraday rotation exhibits a negative code. The linearly polarized light having been rotated passes through an analyzer 15 whose transmission-polarization direction is made different by 45 degrees from that of the polarizer 13, and then converted in the magnitude of the rotation angle  thereof to a change in the intensity of an emitted light 8b. In order to compose this magneto-optical converting section, there is generally used an optical magnetic field sensor composed as shown in Fig. 2 (see National Technical Report Vol. 38, No. 2, P. 127, 1992). In the optical magnetic field sensor composed as shown in Fig. 2, an optical fiber 9 employs multi-mode fibers having a core size 80 µm, and a lens 12 employs self-focussing rod lens having a pitch 0.25. The polarizer 13 and the analyzer 15 employ polarization beam splitters; and a full-reflecting mirror 16 is used to bend an optical path by 90 degrees. The polarization beam splitters and the full-reflecting mirror are cubes whose side each is 5 mm. The magneto-optical element 14 employs a rare-earth iron garnet crystal. A system using the magnetic field measuring equipment to which such principle is applied has been proposed in which magnetic field measuring units are arranged at a plurality of points in a transmission/distribution line; the electrical outputs from each measuring unit are inputted to an arithmetic unit, where the sum of or difference among the outputs with respect to their waveform is taken as a reference signal; and for example, a zero-phase current in the transmission/distribution line is detected to determine an accident. However, where a magneto-optical element used for such optical magnetic field sensor employs a ferrimagnetic rare-earth iron garnet crystal, the magnetic domain specific to the rare-earth iron garnet crystal causes the light having transmitted the crystal to be diffracted. The diffracted light is defined as a zero-order light 24, a first-order light 25, a second-order light 26 and the like on a screen 28 from the center thereof, as shown in Fig. 14. In the optical magnetic field sensor composed as shown in Fig. 2, the detection condition of the diffracted light in the lens 12 on the light emitted side is substantially zero-order light detection, so that the output therefrom is expressed by the following equation (1) (see J. Mag. Soc. Jpn., Vol. 14, No. 4 P.642, 1990): V ⊘ = 1 2 {cos  F + M Ms sin  F } 2 wherein,  F is the Faraday rotation (saturation Faraday rotation) when a material is magnetically saturated and expressed by  F = F·L in which F is a coefficient of the Faraday rotation specific to the material and L is an optical path length (element length); M is a magnetization of the material when a magnetic field is applied to it; and Ms is a magnetization (saturation magnetization) when the material is magnetically saturated. As a magneto-optical element used for the optical magnetic field sensor as described above, there has been disclosed a rare-earth iron garnet crystal which is expressed by a general formula (chemical formula 1), in which the value of X is set at X = 1.3; that of Y at Y = 0.1; that of Z at Z =0.1; and that of W at W = 0.6 (see U.S. Patent No. 5,212,446 or the technical report OQE92-105, 1992, of The Institute of Electronics, Information and Communication Engineers). In the prior art, the substitution of Bi or Gd for Y allows a magneto-optical element having good temperature properties. A chemical formula of the crystal used for prior art examples is shown in (formula 3). Bi x Gd y R z Y 3-(x-y-z) Fe 5-w Ga w O 12 However, where an optical magnetic field sensor is composed, as shown in Fig. 2, by the use of the magneto-optical element, there has been indicated a linearity error of magnetic field measurement of ±2.0% or less within a range 0,40 kA/m to 15.9 kA/m (5.0 0e to 200 0e), as shown in Fig. 4, so that there is a problem in practice with respect to the accuracy of a magnetic field measuring equipment. The linearity error is due to a fact that where the magneto-optical element used for an optical magnetic field sensor employs a Bi-substituted rare-earth iron garnet crystal as described above, when a light is transmitted through the garnet crystal being a ferrimagnetic substance, a light diffraction occurs due to the. magnetic domain structure of the garnet crystal, whereby the diffracted light having transmitted the crystal is not completely detected in an optical system on the light emitted side in which only the zero-order light is thus received. SUMMARY OF THE INVENTION The present invention as defined by claims 1 to 4 respectively is made in view of such points, and it is an object of the invention to improve the measuring accuracy for magnetic field, to compose an optical magnetic field sensor capable of detecting higher order diffracted light, and to provide a magnetic field measuring equipment having a high measuring accuracy. This object is solved with the magnetic field sensor according to claims 1, 2, 3 and 4 and to a magnetic field measuring equipement according to claim 6. The present invention provides a magneto-optical element having a high measuring accuracy in a wider magnetic field range with the the optical magnetic field sensor; and the operation thereof will be explained hereinafter. Equation (1), when expanded, can be expressed as in equation (2). V ⊘ ≒ 1 2 cos 2  F + cos  F sin  F Hs · H + sin 2  F 2Hs 2 · H 2 where M = χH, in which χ is magnetic susceptibility. Considering that the applied magnetic field is alternating current magnetic field, substitution of H = H  sinωt in equation (2) causes the output alternating current component to be expressed as in equation (3). V  ac = A· H · sinωt + B · H  2 · sin (2ωt - π/2) where A and B are constants, and expressed as follows: A = cos F sin F Hs B = sin 2  F 4Hs 2 It will be understood from equation (3) that the equation expressing the output includes the term sin2ωt, and the coefficient thereof is multiplied by the term of the square of magnetic field intensity, H ⊘ 2 . Therefore, for the alternating current magnetic field, the presence of a second higher harmonic with respect to the fundamental harmonic of sinωt causes a distortion in linearity with respect to magnetic field. Accordingly, the larger the applied magnetic field intensity H ⊘ becomes, the larger the amplitude of the second higher harmonic becomes in proportion to the square of H  , so that the linearly of the output becomes worse. On the other hand, where all-order detection is performed so that all lights up to a high-order diffracted light are to be received, the output from the sensor is expressed as in the following equation: V r = 1 2 {1 + M Ms sin 2  F } As seen from equation (4), where all-order detection is performed, it is considered that the effective value of the output will be simply proportional to the applied magnetic field, and that the non-linearity of the output due to the presence of a second higher harmonic will not occur. In such point of view, the present invention makes large the magnetic field in which the magnetic domain structure of a magneto-optical element disappears, that is, the magnetic field Hs which is required to magnetically saturate the garnet crystal by composition control, thereby making small the coefficient B of the second higher harmonic which may cause the distortion in linearity, and allowing the measuring accuracy for the magnetic field to be improved. Further, in an magnetic field sensor comprising optical components, the present invention composes an optical system in such a manner that it receives the light diffracted in the rare-earth iron garnet crystal being a ferrimagnetic substance, including lights up to higher-order ones, thereby allowing the measuring accuracy for the magnetic field to be improved. As apparent from the above description, the present invention makes it possible to provide an optical magnetic field sensor having a higher linearity than with prior art, so that the industrial value of such sensor becomes significant. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a magnetic field measuring equipment using an optical magnetic field sensor according to the present invention. Fig. 2 is a block diagram of a conventional optical magnetic field sensor. Fig. 3 is a typical view showing the measuring principle of a magnetic field using the Faraday effect. Fig. 4 is a graph showing the linearity error of the output of a magnetic field measuring equipment using a conventional magneto-optical element. Fig. 5 is a graph showing the linearity error of the output of a magnetic field measuring equipment using a magneto-optical element according to an embodiment the present invention. Fig. 6 is a graph showing the linearity error of the output of a magnetic field measuring equipment using a magneto-optical element according to an embodiment of the present invention. Fig. 7 is a block diagram of an optical magnetic field sensor not according to the present invention. Fig. 8 is a block diagram of an optical magnetic field sensor according to the present invention. Fig. 9 is a block diagram of an optical magnetic field sensor according to the present invention. Fig. 10 is a block diagram of an optical magnetic field sensor according to the present invention. Fig. 11 is a block diagram of an optical magnetic field sensor according to the present invention. Fig. 12 is a block diagram of an optical magnetic field sensor including embodiments the present invention. Fig. 13 is a graph showing the linearity error of the output of a magnetic field measuring equipment using a magneto-optical element and an optical magnetic field sensor according to the present invention. Fig. 14 is a typical view to help explain a light diffraction phenomenon due to a ferrimagnetic garnet crystal. DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to drawings, embodiments of the present invention will be explained hereinafter. Fig. 1 shows an embodiment of a magnetic field measuring equipment using a magneto-optical element and an optical magnetic field sensor according to the present invention. An optical magnetic field sensor 1 is arranged under a magnetic field to be measured or in the gap of a core 6 as shown in Fig. 1. In Fig. 1, the numeral 3 indicates an optical fiber forming an optical transmission path. The numeral 2 indicates optical signal generation means using an LED having a wavelength 0.8 µm band, or an LD having a wavelength 1.3 µm band or 1.5 µm band. The numeral 4 indicates detection means for detecting a light having passed through the optical magnetic field sensor 1 to convert the light to an electrical signal, which usually employs Ge-PD, Si PIN-PD or the like, but this time employs Si PIN-PD because it uses the LED having a wavelength 0.8 µm band. The numeral 4 indicates a signal processing circuit; and the numeral 7 indicates an electrical wire to be measured The optical magnetic field sensor 1 is conventionally composed as shown in Fig. 2, in which the polarizer 13 provided on one end face of the magneto-optical element 14 and the analyzer 15 provided on the other end face are installed in such a manner that their transmission-polarization directions are crossed 45 degrees to each other. The angle, even if it deviates somewhat from 45 degrees, will not significantly affect the linearity properties of the sensor. The polarizer 13 and the analyzer 15 often employ polarization beam splitters. The numeral 12 indicates a lens for turning a light radiated to the magneto-optical element to a collimated light, or for turning a light having transmitted the magneto-optical element to a converged light. First, an example of an improved composition of the magneto-optical element will be explained, which is part of an inventive magnetic field measuring equipment according to claim 7. Using a Bi 2 O 3 -PbO-B 2 O 5 based flux, the Bi-substituted rare-earth iron garnet crystal expressed by a general formula (formula 1) was caused to be crystallizedly grown on a Ca-Mg-Zr-substituted Gd 3 Ga 5 O 12 substrate with the LPB method. Table 1 shows the example. The numerical values indicate the composition ratio for each element. Table 1 also shows the results of the measurement of the linearity error within a magnetic field range 0 A/m to 15,9 kA/m (0 0e to 200 0e), with the magneto-optical elements thus produced used for the optical magnetic field sensor of Fig. 2. The frequency of the alternating current magnetic field is 60 Hz. The samples of the linearity properties marked with # in Table 1 are beyond the scope of the present invention. Sample No. Composition of rare-earth iron garnet crystal Linearity error (%) of sensor output (0.40 ∼ 15.9) kA/m (5.0 ∼ 2000e) Bi Gd La Y Fe Ga X Y Z 3 -X-Y-Z 5-W W 1 0.8 0.2 0.9 1.1 4.7 0.3 ≦ ± 1.0% 2 0.8 0.2 0.9 1.1 4.8 0.2 ≦ ± 0.8% 3 0.8 0.2 0.7 1.3 4.9 0.1 ≦ ± 0.6% 4 1.0 0.3 0.8 0.9 4.9 0.1 ≦ ± 0.6% * 5 1.3 0.4 0.1 1.2 5.0 0.0 ≦ ± 0.5% 6 1.3 0.1 0.1 1.5 4.6 0.4 # ≦ ± 1.5% 7 1.3 0.1 0.1 1.5 4.4 0.6 # ≦ ± 2.0% For No. 5 sample marked with * in Table 1, measured data of the linearity with respect to magnetic field is shown in Fig. 5. Although the improved magneto-optical element is expressed.by a general formula (formula 1), where its crystal composition range is such that the value of X is set at 0.8 ≤ X ≤ 1.3; that of Y at 0.2≦ Y ≦ 0.4 that of Z at 0 .1 ≤ Z≤S 0.9; and that of W at 0≦Z≦0.3; the linearity error is ±1.0% or less within a magnetic field range 0,40 kA/m to 15,9 kA/m (5.0 0e to 200 0e). It will be well understood that comparing with Fig. 4 of prior art example, the linearity is improved. For No. 5 sample in Table 1 and the above-mentioned prior art example, the results of the measurement of the magnetic field Hs required to magnetically saturate the magneto-optical element at room temperature are shown in Table 2: Hs kA/m (Oe) Prior art example (BiGdLaY) 3 (FeGa) 5 0 12 57,3 (720) Example (BiGdLaY) 3 Fe 5 0 12 115,4 (1450) These results show that the ferrimagnetic rare-earth iron garnet crystal in which a non-magnetic element is not substituted for Fe element, compared with that in which the non-magnetic element is otherwise, has a higher Hs and a smaller coefficient B shown in equation 3, thereby improving the linearity with respect to magnetic field. Although the example has described the alternating current magnetic field, for the direct current magnetic field, a similar consideration may be made such that for the optical magnetic field sensor having the composition as shown in Fig. 2, the larger Hs the magneto-optical element employs, the more the linearity with respect to magnetic field is improved. [Example 2] The magneto-optical elements of Table 1 shown are those in which Gd element has been substituted in order to consider also the temperature properties. Therefore, with these magneto-optical elements used for the magnetic field measuring equipment, a temperature variation in their outputs was evaluated within a temperature range -20°C to +80°C, with the results that the linearity error is ± 1.0% or less for all magneto-optical elements. On the other hand, where the temperature properties are not considered, the magneto-optical element, shown by a general formula (chemical formula 2) in which Gd element is not substituted, may also be used as an element having a good linearity for the magnetic field measuring equipment. (Bi x R y Y 3-x-y ) Fe 5 O 12 Where a magneto-optical element expressed by a general formula (chemical formula 2) is produced, and used to compose a magnetic field measuring equipment as shown in Fig. 1, the measured results of the linearity error are shown in Table 3. The numerical values indicate the composition ratio for each element. Sample No. Composition of garnet crystal Linearity error (%) of sensor output 0,40 kA/m∼15,9 kA/m (5.0∼2000e) Bi La Y X Y 3-X-Y 1 0.6 0.2 2.2 ≦ ± 1.0% 2 0.8 0.2 2.0 ≦ ± 1.0% 3 1.0 0.1 1.9 ≦ ± 1.0% * 4 1.3 0.1 1.6 ≦ ± 1.0% For No. 4 sample marked with * in Table 3, measured data of the linearity with respect to magnetic field is shown in Fig. 6. Although the magneto-optical element is expressed by a general formula (formula 1), where its crystal composition range is such that the value of X is set at 0.6≦X≦1.3; and that of Y at ≦0.15≤ Y≤0.2, the linearity error is ±1.0% or less within the magnetic field range 0,40 kA/m to 15,9 kA/m (5.0 0e to 200 0e), so that the temperature properties are not good, but the linearity error with respect to magnetic field indicates sufficiently usable properties. Although the present example 2 has described an example using La as a substituting element in order to obtain a lattice matching, one or more kinds of rare-earth elements could also have been used for R elements in a general formula (formula 1) or (formula 2). In that case, good results are obtained by the use of a non-magnetic element which does not affect the saturation magnetization of the Bi-substituted rare-earth iron garnet crystal. Even where those other than a Ca-Mg-Zr-substituted Gd 3 Ga 5 O 12 crystal substrate having different lattice constants were used for a garnet crystal substrate, one or more kinds of rare-earth elements were substituted for the R elements in a general formula (formula 1) or (formula 2) to obtain a lattice matching, whereby a Bi-substituted rare-earth iron garnet crystal having a good linearity could be grown. Further, in order to compensate the temperature properties, the elements such as Tb, Dy, Ho, Er and Tm also were confirmed to be effective. Although in the above case, a magneto-optical element was caused to be grown on a Ca-Mg-Zr-substituted Gd 3 Ga 5 O 12 crystalline substrate, the element was caused to be grown also on a Nd 3 Ga 5 O 12 substrate by changing of growth conditions, whereby similar results were obtained. By the use of the chemical vapor deposition method, the element may be caused to be epitaxially grown on these substrates. However, although a magneto-optical element which is expressed by a general formula (formula 1), and whose crystal composition range is such that the value of X is set at 0.8≦X≦1.3; that of Y at 0. 2 ≦ Y ≦ 0. 4; that of Z at 0. 1 ≦ Z ≦ 0.9; and that of W at 0 ≦ W≦0.3; and a magneto-optical element which is expressed by a general formula (formula 2), and whose crystal composition range is such that the value of X is set at 0.6≦X≦1.3; and that of Y at 0.1≦Y ≦ 0.2 may be formed as a polycrystal line substance without being epitaxially grown depending on growth conditions in either of the growth methods, even such magneto-optical element as a polycrystal line substance has a somewhat-increased light absorption loss, but may be sufficiently usable. An embodiment in which the linearity was improved by an optical system of an optical magnetic field sensor will be explained hereinafter. Exemple 3 useful for the understanding of the invention Fig. 7 is a block diagram showing an optical magnetic field sensor according to an exemple not according the present invention but useful for the understanding. In Fig. 7, the optical fiber 9 on the light radiated side employs multi mode fibers having an optical fiber core diameter 50 µm, while an optical fiber 17 on the light emitted side employs multi mode fibers having an optical fiber core diameter 200 µm. The lenses on the light radiated and emitted sides are self-focussing rod lenses having a pitch 0.25. The polarizer 13 and the analyzer 15 are arranged on both the oppositing faces of the rare-earth iron garnet crystal in such a manner that their transmission-polarization directions are crossed 45 degrees to each other. The full-reflecting mirror 16 is used to bend an optical path by 90 degrees. The light 8a radiated from a light source passes through the optical fiber 9, is turned by the lens 12 to a collimated light, then turned by the polarizer 13 to a linearly polarized light, and then the light 8a thus reflected is radiated to the magneto-optical element 14. At this point, the beam diameter of the light radiated to the magneto-optical element 14 is 800 µm. The light having been radiated to the magneto-optical element 14 is diffracted by the magneto-optical element 14 having a multi-magnetic domain structure, as shown in Fig. 14, propagates while diverging, is light-detected only for 45-degree component with respect to the polarizer 13 by the analyzer 15, and then bent by 90 degrees in the propagating direction by the full-reflecting mirror 16. The light bent by 90 degrees in the propagating direction is radiated to the lens to be converged on an optical fiber 18 on the light output side. Since the rare-earth iron garnet crystal employs the (BiGdLaY) 3 (FeGa) 5 O 12 crystal, the diffraction angle  per one order of diffracted light for wavelength 880 µm is 2.6 degrees. As shown in the example 3 ,where the multi-mode fibers having a core diameter 200 µm larger than the core diameter 50 µm of the optical fiber 9 on the light input side are used for the optical fiber 17 on the light output side, the numerical aperture of the optical system on the light output side becomes larger than the numerical aperture of the optical system on the light input side. Therefore, where the optical fiber having the same core diameter is used for the optical fiber 9 to compose the optical magnetic field sensor as shown in Fig. 2 of prior art example, only the zero-order light has been detected, while in the present example 3, the order number of the diffracted light which the lens 7 on the light output side can detect has become large, thereby making it possible to detect a light up to a higher-order light. [Embodiment 1] A first embodiment of an optical magnetic field sensor according to the invention is shown in Fig. 8. The point that the optical magnetic field sensor of Fig. 8 is different from the prior art example shown in Fig. 2 exists in that self-focussing rod lenses having a diameter 1 mm and a pitch 0.25 are arranged on a lens 18 on the light input side, and self-focussing rod lenses having a diameter 3 mm and 5 mm, and a pitch 0.25 are arranged on a lens 19 on the light output side. As shown in the present embodiment, where the lens 19 on the light output side employs lenses having a larger diameter than that of lenses for the lens 18 on the light input side, similarly to example 3, the numerical aperture of the optical system on the light output side becomes larger than the numerical aperture of the optical system on the light input side. Therefore, where the self-focussing rod lenses having the same core diameter and a pitch 0.25 are used for the lenses on the light input and output sides to compose the optical magnetic field sensor as shown in Fig. 2 of prior art example, only the zero-order light has been detected, While in the first embodiment, the order number of the diffracted light which the lens 19 on the light output side can detect has become large, thereby making it possible to detect a light up to a higher-order light. The same effect has also been found where an aspherical lens and a spherical lens capable of obtaining a collimated light are used instead of the self-focussing rod lenses 18, 19 of Fig. 8 so that the diameter of the lens on the light output side is made larger than that of the lens on the light input side. [Embodiment 2] A second embodiment is shown in Fig. 9. The point that the optical magnetic field sensor of Fig. 9 is different from the prior art example shown in Fig. 2 exists in that in the optical magnetic field sensor of prior art example, a Fresnel lens 20 is arranged on the plane opposite to the polarizer 13 with respect to the magneto-optical element 14 on the optical path in the direction shown in Fig. 9. The Fresnel lens employs lenses having a diameter 3 mm and a numerical aperture 0.3. Therefore, the total zone number of the Fresnel lens used for the wave length 880nm is in fact about 2.56x10 5 . As shown in the present embodiment, where the Fresnel lens 20 is arranged directly behind the magneto-optical element 14 with respect to the light propagating direction so as to turn the deverging light to a collimated light, the light diffracted by the magneto-optical element 14 can be efficiently turned to a collimated light. Therefore, where the optical magnetic field sensor is composed as shown in Fig. 2 of prior art example, only the zero-order light has been detected, while in the second embodiment, the order number of the diffracted light which the lens 12 on the light output side can detect has become large, thereby making it possible to detect a light up to a higher-order light. The Fresnel lens 20 has an advantage in that the lens is thinner in lens thickness than that of a flat convex lens, so that when the optical magnetic field sensor is housed in a case, the external shape of the sensor becomes not large. The same effect has been found that a grating lens is used instead of the Fresnel lens [Embodiment 3] A third embodiment is shown in Fig. 10. The point that the optical magnetic field sensor of Fig. 10 is different from the prior art example shown in Fig. 2 exists in that spherical lenses 21 having a diameter 3 mm are arranged on lenses on the light input and output sides to compose a light-converging optical system. Now, when the light-converging optical system is to be composed, care must be taken on a distance c between an optical fiber 9a on the light input side and a lens 21a on the light input side, a distance d between a lens 21a on the light input side and the polarizer 13, a distance e between the full-deflecting mirror 16 and a lens 21b on the light output side, and a distance f between the lens 21b on the light output side and an optical fiber 9b on the light output side, on the optical path, so that in the present embodiment, all the respective distances have been designed to be 1 mm to 1.4 mm. Meanwhile the values of the diameter of the spherical lense , c , d , e , f , are selected in a manner that the optical insertion loss is made minimum. Now in that case, the polarizer, the light detector and the full-reflecting mirror are cubes whose side each is 5 mm. Where the material of the spherical lens employs BK7, and the light-source wavelength employs 880nm, the beam diameter of a light at a crystal position is 480 µm. Where a light-converging optical system'is composed as shown in the present embodiment, the crystal position is located near a beam waist, so that the beam diameter of a light at the crystal position has become smaller than that with prior art. As a result, the lens 21b on the light output side has made it possible to detect a light up to a higher-order light. [Embodiment 4] A fourth embodiment is shown in Fig. 11. The point that the optical magnetic field sensor of Fig. 11 is different from the prior art example shown in Fig. 2 exists in that a magneto-optical element 22 is arranged in a manner to be inclined by an angle α to the light propagating direction. The arrangement of the magneto-optical element 22 so as to be inclined to the light propagating direction allows a diffraction effect developing in the magneto-optical element to be reduced. The rare-earth iron garnet crystal being a ferrimagnetic substance has a multi-magnetic domain structure, so that the crystal becomes a diffraction grit with respect to light. Generally, when the light incident angle to the diffraction grit is caused to be increased from zero degree, the diffraction effect of the diffraction grit is reduced extremely. Therefore, the magneto -optical element 22 is caused to be inclined to the light propagating direction of the incident beam as shown in the present embodiment, thereby allowing the diffraction effect of the magneto-optical element to be reduced and a measuring accuracy to be improved. In the present embodiment, from the sensor sensitivity and the light insertion loss points of view, the inclined angel a has been made an optimum value 45 degrees. [Embodiment 5] A fifth embodiment is shown in Fig. 12. Fig. 12 shows a linear optical magnetic field sensor, in which the optical fiber 9a on the light input side is multi-mode fibers having a core diameter 200 µm. The polarizer 13 and the analyzer 15 are laminated polarizer thin-films, and installed on both faces of the magneto-optical element 14 in such a manner that their transmission-polarization directions are crossed 45 degrees to each other. The magneto-optical element 14, the polarizer 13 and the analyzer 15 used are 3 x 3 mm square in size. The numeral 12 indicates a lens, and the optical fiber 9b on the light output side is multi-mode fibers having a core diameter 400 µm. The numeral 11 indicates a ferrule to fix fibers. These components are housed in a case 23. The optical magnetic field sensor shown in Fig. 12 comprises, in combination, the above-mentioned example 3 and embodiments 3 through 7. If a transmitted light can be converged on the end face of the optical fiber on the light output side, the lens 12 may employ a concave lens or the like. Where the core diameter of the optical fiber on the light output side is larger than that of the optical fiber on the light input side, a composition in which the lens 12 is not arranged has been realized. Then, in order to evaluate the optical magnetic field sensors of Figs. 7 through 12, a current measuring equipment is composed as shown in Fig. 1 to measure the linearity error. The optical magnetic field sensor 1 is installed directly to the electric wire or arranged in the gap of the core as shown in Fig. 1. Table 4 concludes the results that using an optical magnetic field sensor for the current measuring equipment composed as described above, the linearity error was measured within a magnetic field range 0 kA/m to 15,9 kA/m (0e to 200 0e). The frequency of the alternating current used is 60 Hz. Compared with the measured data of prior art examples, it will be understood from the results that the linearity is improved by these optical system compositions. Therefore, the composition of the optical magnetic field sensor in these optical systems has allowed a diffracted light up to a higher-order light to be detected, the signal intensity of a second higher harmonic included in the output to be reduced, and the linearity of the optical magnetic field sensor with respect to magnetic field to be significantly improved. Linearity error (%) 0,40 kA/m ∼ 15,9 kA/m (5.0 ∼ 200 Oe) Prior art example ≦ ± 2.0 Example 3 ≦ ± 1.0 Embodiment 4 ≦ ± 1.0 Embodiment 5 ≦ ± 1.5 Embodiment 6 ≦ ± 1.5 Embodiment 7 ≦ ± 1.0 Embodiment 8 ≦ ± 1.0 Therefore, where using the optical magnetic field sensor according to the optical system of the present invention, a current measuring equipment is composed, an optical CT can be composed which has a ratio error of ± 1% or less within a current measurement range 0.025 I to I with respect to a rated current value I. Although in the optical magnetic field sensor of the present embodiment, the polarizer 13 and the analyzer 15 have been made polarization beam splitters, they may employ a glass-made polarized plate or. a laminated polarizer thin-film. Particularly, an optical magnetic field sensor in which a glass-made polarized plate or an laminated polarizer thin-film is used for the polarizer 13 and the analyzer 15 has a big advantage of allowing miniaturization. Also, a lens capable of obtaining a collimated light can be substituted for the self-focussing rod lens. Further, a linear optical magnetic field sensor has been composed without using the full-reflecting mirror. The improvement in linearity has been found not only for a light source having a 0.8 µm band, but also for other wavelengths of 1.3 µm band and 1.5 µm band passing through the rare-earth iron garnet crystal. It has also been confirmed that a magnetic field can be measured with a good linearity not only for a frequency 60 Hz, but also for a range from direct current magnetic field to about several-hundred KHz. Although there have been shown examples of optical magnetic field sensors in which the rare-earth iron garnet crystal expressed by (formula 1) was used in order to consider the temperature properties of the optical magnetic field sensors, the composition of the optical system of the present embodiment can be applied to all optical magnetic field sensors in which the garnet crystal being a ferrimagnetic substance is used for a magneto-optical element; and also an optical magnetic field sensor can comprise, in combination, these embodiments and exemples. Fig. 13 shows the results of the linearity error for a magnetic field measuring equipment composed in such a manner that a magneto-optical element expressed by (formula 1) is used for the optical magnetic field sensor using the light-collecting optical system of the embodiment 3. It will be understood from Fig. 13 that the linearity is obviously improved by the crystal composition and the optical system.

Claims: optical magnetic field sensor in which a polarizer (13), a magneto-optical element (14) and an analyser (15) whose transmission-polarization direction is made different from that of said polarizer (13) are arranged along at least a light propagating direction to detect as an output light intensity a magnetic field to be measured characterized in that a lens diameter of a second lens (19) provided with respect to the light propogating direction behind said analyser (15) at an end of said magneto-optical element is larger than a lens diameter of a first lens (18) provided with respect to the light propogating direction in front of said polarizer (13) at the other end of said magneto-optical element, to enable detection of higher-order diffracted light (25, 26, 27) besides zero-order light (24). Optical magnetic field sensor in which a polarizer (13), a magneto-optical element (14) and an analyser (15) whose transmission-polarization direction is made different from that of said polarizer (13) are arranged along at least a light propagating direction to detect as an output light intensity a magnetic field to be measured, characterized in that a Fresnel lens(20) or a grating lens is provided on an optical path between said magneto-optical element (14) and said analyser (15) in such a direction in which a diverged diffracted light which passes through the magneto-optical element is made a collimated light, to enable detection of higher-order diffracted light (25, 26, 27) besides zero-order light (24). Optical magnetic field sensor in which a polarizer (13), a magneto-optical element (14) and an analyser (15) whose transmission-polarization direction is made different from that of said polarizer (13) are arranged along at least a light propagating direction to detect as an output light intensity a magnetic field to be measured, characterized in that a first lens (21a) is provided with respect to the light propagating direction in front of said polarizer (13) at an end of said magneto-optical element and a second lens (21b) is provided with respect to the light propagating direction behind said analyser (15) at the other end of said magneto-optical element forming a light-converging optical system, said first and second lens each converging the light in the light propagating direction, to enable detection of higher-order diffracted light (25, 26, 27) besides zero-order light (24). Optical magnetic field sensor in which a polarizer (13), a magneto-optical element (14) and an analyser (15) whose transmission-polarization direction is made different from that of said polarizer (13) are arranged along at least a light propagating direction to detect as an output light intensity a magnetic field to be measured, characterized in that said magneto-optical element (14) is arranged in a manner to be inclined at 45 degrees with respect to the light propagating direction. An optical magnetic field sensor in accordance with claims 1, 2, 3 or 4 wherein the magneto-optical element (14) is of a ferromagnetic rare-earth iron garnet crystal. A magnetic field measuring equipment comprising: the optical magnetic field sensor in accordance with claims 1, 2, 3 or 4; a first optical transmission path (9, 9a) which is provided with respect to the light propagating direction in front of said polarizer (13) at an end of said magneto-optical element (14); light generation means (1) for radiating a light into said first optical transmission path (9, 9a); a second optical transmission path (9b, 9) which is provided with respect to the light propagating direction behind said analyser (15) at the other end of said magneto-optical element; light detection means (4) for detecting an optical output from said second optical transmission path (9, 9b) to convert the output to an electrical signal; and a signal process section (5) for processing the electrical signal from said light detection means (4), wherein at least the magneto-optical element is arranged in a magnetic field, said signal process section (5) detects a change in output light intensity developed by a transmission of said radiated light through said magneto-optical element , whereby a magnetic field intensity can be measured. The magnetic field measuring equipment in accordance with claim 6, wherein the magneto-optical element (14) has the chemical formula (1): (Bi x Gd y R z Y 3-x-y-z ) (Fe 5-w Ga w ) O 12 or the formula (2): (Bi x R y Y 3-x-y ) Fe 5 O 12 .

Inventors: ITOH NOBUKI, ITOH, NOBUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: G01R33
IPC codes: G01R33